1. Field of the Invention
The present invention relates to a fluid flow rate measuring apparatus used at a location requiring measurement of a flow rate of fluid such as air, for example, at an engine control device in a motor vehicle or an air conditioning appliance.
2. Description of the Prior Art
FIG. 13 is a fragmentary sectional view of a conventional fluid flow rate measuring apparatus disclosed in Japanese Patent Laid-Open Publication No. 11-326003(1999). The conventional fluid flow rate measuring apparatus includes a silicon substrate 101, an air space 102 defined in the silicon substrate 101 by etching, thin film members, i.e., thin-wall portions 103 and 104 bridged over the air space 102, first and second heating elements 105 and 106 and first and second temperature detecting elements 107 and 108. The heating elements 105 and 106 and the temperature detecting elements 107 and 108 are made of a temperature sensitive resistance material whose resistance value varies according to temperature. For example, platinum is used as the temperature sensitive resistance material. The first and second heating elements 105 and 106 are manufactured so as to have substantially identical resistance values and temperature coefficients. The first and second temperature detecting elements 107 and 108 are also manufactured so as to have substantially identical resistance values and temperature coefficients.
In FIG. 13, the first heating element 105 and the first temperature detecting element 107 are spaced away from each other in order to facilitate understanding of their arrangements but are actually formed at substantially identical locations so as to be held in close contact with each other thermally. Likewise, the second heating element 106 and the second temperature detecting element 108 are spaced away from each other in order to facilitate understanding of their arrangements but are actually formed at substantially identical locations so as to be held in close contact with each other thermally.
FIG. 14 shows a circuit of the conventional fluid flow rate measuring apparatus of FIG. 13. The circuit includes fixed resistances 109 and 110 which form a bridge circuit 117 with the first and second temperature detecting elements 107 and 108, a comparator 111 for comparing intermediate potentials 118 and 119 of the bridge circuit 117, an inverter 112, electronic switches 113 and 114, a power source 115 and a fluid flow path 116. The conventional circuit is operated as follows. When a difference between the intermediate potentials 118 and 119 is produced in case there is no flow of fluid, the comparator 111 detects this difference between the intermediate potentials 118 and 119 so as to control the electronic switches 113 and 114. If the fixed resistances 109 and 110 are set to have an identical resistance value, the first and second temperature detecting elements 107 and 108 also have an identical resistance value and thus, have an identical temperature. In case there is no flow of the fluid, on-state periods of the electronic switches 113 and 114 become identical with each other and thus the ratio of electric power supplied to the first heating element 105 and to the second heating element 106 is 50%:50%.
Subsequently, a case in which the fluid is flowing is described. When the fluid flows in the direction of the arrow in FIG. 14, heat is transferred from the first heating element 105 and the first temperature detecting element 107 to the fluid, so that a temperature of the first temperature detecting element 107 drops. The fluid which absorbed heat from the first heating element 105 and the first temperature detecting element 107 at an upstream side transfers the heat to the second temperature detecting element 108 and thus, a temperature of the second temperature detecting element 108 rises. Therefore, the intermediate potential 118 becomes lower than the intermediate potential 119 and thus, an output of the comparator 111 is at high level. Accordingly, the electronic switch 113 is turned on and thus, electric current flows through the first heating element 105. As a result, the first heating element 105 is heated by Joule heat so as to raise the temperature of the first temperature detecting element 107. Since the first heating element 105 and the first temperature detecting element 107 are cooled by the fluid flow, an on-state period of the electronic switch 113, which should elapse before the intermediate potential 118 exceeds the intermediate potential 119, becomes longer than that of a case in which there is no flow of the fluid. At the time the intermediate potential 118 has risen so as to exceed the intermediate potential 119, the electronic switch 114 is turned on and thus, electric current flows through the second heating element 106. Therefore, the second heating element 106 is heated by Joule heat so as to raise the temperature of the second temperature detecting element 108 and thus, the intermediate potential 119 rises. Since the second heating element 106 and the second temperature detecting element 108 are warmed by the fluid flow, an on-state period of the electronic switch 114, which should elapse before the intermediate potential 119 exceeds the intermediate potential 118, becomes shorter than that of the case in which there is no flow of the fluid. At the time the intermediate potential 119 has exceeded the intermediate potential 118, the electronic switch 114 is turned off and the electronic switch 113 is turned on, so that electric current flows through the first heating element 105 again.
By repeating the above mentioned operations, the intermediate potentials 118 and 119 ire held equally again. Therefore, even if there is a flow of the fluid, the temperatures of the first and second temperature detecting elements 107 and 108 are controlled equally. At this time, quantity of electric power supplied to the first heating element 105 becomes larger than that supplied to the second heating element 106. For example the ratio of the quantity of electric power supplied to the first heating element 105 and to the second heating element 106 is 60%:40%.
FIG. 15 shows an output waveform in the above mentioned operations of the conventional fluid flow rate measuring apparatus of FIG. 13. An output voltage Vout in FIG. 14 has a pulse waveform shown in FIG. 15. As a flow rate of the fluid rises further, quantity of electric power supplied to the first heating element 105 is increased more. Hence, in FIG. 15, an interval t1 increases and an interval t2 decreases. Therefore, if a difference d of duty ratios is measured by using the following equation (1), an output dependent on the flow rate can be obtained.
d=(t1xe2x88x92t2)/(t1+t2)xe2x80x83xe2x80x83(1) 
FIG. 16 having an ordinate representing output and an abscissa representing flow rate shows such output characteristics. Furthermore, the difference d of duty ratios in the equation (1) can be expressed as follows by using a heat release value P1 of the first heating element 105 and a heat release value P2 of the second heating element 106.
(t1xe2x88x92t2)/(t1+t2)=(P1xe2x88x92P2)/(P1+P2)xe2x80x83xe2x80x83(2) 
In this technique, if a back flow occurs, the interval t1 decreases and the interval t2 increases, so that the output is inverted and thus, it is possible to detect the back flow.
FIG. 17 shows dependency of temperature distribution on flow rate in the conventional fluid flow rate measuring apparatus of FIG. 13. In FIG. 17, flow rates v1, v2 and v3 have the relation of (0 less than v1 less than v2 less than v3). Temperature drop of the first temperature detecting element 107 caused by increase of the flow rate is larger than temperature rise of the second temperature detecting element 108. Therefore, if the temperatures of the first and second temperature detecting elements 107 and 108 are controlled equally, absolute values of the temperatures of the first and second temperature detecting elements 107 and 108 will decrease upon increase of the flow rate. Then, the temperatures of the first and second temperature detecting elements 107 and 108 come closest to a temperature of the fluid. When the temperatures of the first and second temperature detecting elements 107 and 108 have become substantially identical with the temperature of the fluid, heat supplied from the first and second heating elements 105 and 106 does not affect the temperatures of the first and second temperature detecting elements 107 and 108, so that it becomes impossible to detect the flow rate of the fluid. Supposing that this flow rate is referred to as a xe2x80x9csaturated flow ratexe2x80x9d, a measurable upper limit of the flow rate in the conventional fluid flow rate measuring apparatus is the saturated flow rate.
FIG. 18 shows relation between temperature changes of the first and second temperature detecting elements 107 and 108 and flow rate in the conventional fluid flow rate measuring apparatus of FIG. 13. In FIG. 18, lines 121 and 122 indicate temperatures of the first and second temperature detecting elements 107 and 108, respectively in the case where the flow rate of the fluid is increased without changing duty ratios of power supply to the first and second heating elements 105 and 106, which duty ratios are obtained when the flow rate is zero. As indicated by the line 121, the temperature of the first temperature detecting element 107 drops upon increase of the flow rate. Meanwhile, as indicated by the line 122, the temperature of the second temperature detecting element 108 rises by absorbing heat from the upstream side in a region of small flow rate but drops from a point. If the first and second temperature detecting elements 107 and 108 which have such dependency on flow rate are subjected to isothermal control, the temperature of the first temperature detecting element 107 rises and the temperature of the second temperature detecting element 108 drops, so that the temperatures of the first and second temperature detecting elements 107 and 108 are converged to a temperature indicated by a line 123. Temperature drop for lowering the temperature of the second temperature detecting element 108 from the line 122 to the line 123 increases in the region of small flow rate as indicated by a sequence of the arrows A and B but decreases in a region of middle flow rate or more as indicated by a sequence of the arrows B, C and D. Hence, since change of a heat dissipation value of the second heating element 106 decreases, change of the difference d of duty ratios in the equation (1) also decreases. Therefore, the conventional fluid flow rate measuring apparatus has such a disadvantage that sensitivity drops also before the flow rate reaches the saturated flow rate.
Accordingly, an essential object of the present invention is to provide, with a view to eliminating the above mentioned drawbacks of prior art, a fluid flow rate measuring apparatus which has high sensitivity over a whole measurable flow rate region.
In order to accomplish this object of the present invention, a fluid flow rate measuring apparatus for measuring a flow rate of fluid, according to the present invention includes first and second heating elements which are disposed at upstream and downstream sides in a direction of flow of the fluid, respectively. First and second temperature detecting element are formed in the vicinity of the first and second heating elements, respectively. A power source is connected to the first and second heating elements and supplies electric power to the first and second heating elements so as to make a temperature of the first temperature detecting element higher by a predetermined value than that of the second temperature detecting element at all times such that the flow rate of the fluid is measured from a ratio of a quantity of the electric power supplied to the first heating element to that supplied to the second heating element.